(1) Field of the Invention
The present invention relates to subcarrier frequency multiplexed optical transmission systems, and more particularly to an optical transmitter for subcarrier frequency multiplexed optical transmission systems, such as for cable television (CATV) systems.
(2) Description of the Related Art
In recent years, cable television (CATV) systems in which digital subcarrier signals are transmitted via optical fibre for new services such as bi-directional image transmission in addition to analog subcarrier signals for conventional television (TV) broadcast services, are attracting attentions. Such analog/digital hybrid optical transmission CATV is reported in detail in, for instance, Yoneda, "Status of Development of Optical Subscriber Transmission Systems", the Optical Communication Symposium of the Institute of Electronics and Communication Engineers of Japan, 1993, OCS93-8S.
Where a number of such frequency multiplexed subcarriers are simultaneously optically transmitted, it is necessary to note a phenomenon of clipping of the output light waveform that is caused when modulation signal inputted to an LD (Laser Diode) as transmission light source momentarily becomes lower than a threshold level. Particularly, where analog subcarrier signals adopting vestigial sideband amplitude modulation (VSB-AM) are transmitted for conventional TV broadcast services, the phenomenon is highly possible because it is necessary to set a high optical modulation degree to ensure a large carrier-to-noise ratio (CNR). When the clipping occurs, momentary distortion (i.e., impulse distortion) is generated to greatly deteriorate the quality of digital subcarrier signal transmission. Effects of the clipping on the digital subcarrier signal transmission are detailed in, for instance, Maeda et al, "Analysis of 16QAM-signal's BER in AM/16QAM Hybrid Optical Transmission", the Transactions of the Institute of Electronics and Communication Engineers of Japan, Spring 1993, B-997.